Injury of nerve tissue is mainly caused by car accidents emerged from the development of the transportation, cancer according to the environmental pollution and side effects of the operations related to nerve system (B. Schlosshauer et al., “Rat Schwann cells inbioresorbable nerve guides to promote and accelerate axonal regeneration,” Brain Res., 963, 321-326, 2003). The injury of nerve tissue results in incapacity of a motor skill and sensory paralysis of the muscle which used to be controlled by the injured nerve.
In order to treat the symptom of nerve injury, a method connecting severed nerve to nerve and a method connecting injured tissue to a normal nerve obtained from the body part which is hardly used or less important have been mainly used. Regarding these methods, the method connecting nerve to nerve directly can be only applied to the nerve tissue having minor injury or very short length. This method cannot be applied to the specific length of nerve which is often treated in the practical operations. For the method autografting normal nerve tissue obtained from the body part to the injured nerve tissue, another nerve injury can be occurred in the body part from which the normal nerve tissue was removed. Also, this method has difficulty for fitting the thickness and shape of the autografted nerve tissue into those of the injured nerve tissue. According to the clinical results which are reported in connection with these methods, five years later from when these methods were used, the patients' capacity for a motor skill was recovered 25% and the sensory function was recovered only 1-3%. In this regard, the usefulness of these methods is not trustworthy. (A. C. Lee et al., “Controlled release of nerve growth factor enhances sciatic nerve regeneration,” Exp. Neurol., 184, 295-303, 2003). Recently, in order to replace these methods, a lot of researches have been studied with respect to injured nerve and nerve conduit which helps nerves to perform normal nerve function.
A nerve conduit is a tube inducing nerve connection to the inside of an artificial tube to which the tip of severed nerves is fixed. The method for using the nerve conduit has some advantages. First, the method can block penetration of scar tissue which disturbs nerve generation from the environment. Second, the method can lead the growth of neuraxis (an essential constituent of nerve for communication) to appropriate direction. Last, the method can help regeneration accelerator substances which are secreted from nerve itself to be inside of the tube, while the substances which disturb nerve generation are blocked from the outside.
For the materials of nerve conduit, body tissues (e.g., vein, epineurium, muscle tissue removed of cell, etc.), natural polymers (e.g., collagen, chitosan, etc.), synthetic polymers (e.g., silicon, polylactic acid (PLA), polyglycolic acid (PGA), polylactic acid-co-glycolic acid (PLGA), polycaprolactone, poly lactic acid-co-caprolactone, poly hydroxybutyric acid-co-hydroxyvaleric acid, poly phosphoester, etc.) have been mainly researched. However, if body tissues are used as the materials of nerve conduit, the supply of the tissues are not enough and if body tissues are not obtained by autograft, the usage of nerve conduit is limited due to an immune reaction. Also, if natural polymers are used, the polymers are easily absorbed in the body and the properties are hard to control. Accordingly, natural polymers have many unsolved problems to be used as a material of nerve conduit. Recently, synthetic polymers which have a plurality of supply chains and excellent properties have been primarily studied as the materials for the nerve conduit.
For nerve conduit, it is essential to meet the following requirements. First, inner space of nerve conduit has to be maintained during regeneration and restoration of nerve tissue. Second, nerve conduit has to have proper elasticity and tensile strength in order to keep the sutures of nerve endings stable with the movement of the surgical region. Third, nerve conduit has to be biodegradable according to the regeneration stage of the nerve tissue in order to reduce additional removal procedure. Fourth, tissue rejection against nerve conduit has to be avoided or minimized. Fifth, nerve conduit does not have infectious factors such as virus or bacteria, etc. Sixth, nerve conduit needs to have proper permeability to exchange body fluid in and out of nerve conduit. Seventh, breakdown products of nerve conduit need to have non-toxic properties.
In clinics, non-degradable silicon tube is commonly used, but since this tube is remained in the body even though nerves are regenerated, it causes disadvantages such as chronic inflammation, calcium products of silicon tube and several pains originated from the pressed regenerated nerves; therefore, use thereof has been gradually decreasing. Recently, nerve conduit comprising biodegradable polymers, which melt in the body, has been developed and applied in clinics (e.g., Neurotuve-® by Neuroregen, LLC, USA). This is made of polyglycolic acid (PGA) which is typically used as surgical suture and developed for restoration of isolated sensory nerve within 3 cm of length. The outside of this product has been reformed to a woven structure to maintain inner shape of the product against bending.
Meanwhile, a nerve conduit comprising the same PGA has been developed in Japan for longer isolated nerves, wherein inside of the nerve conduit is filled with collagen sponge which has high-tissue affinity and outside of the nerve conduit is coated with collagen solution several times. As a result of animal testing regarding this nerve conduit, it was confirmed that the isolated peripheral nerve which was 8 cm length was perfectly restored, and the nerve conduit has been used in clinic since 2002 (Korean Registered Patent No. 10-0718073).
Among the currently available nerve conduits in the market, although a collagen-based nerve conduit has excellent biodegradability, the nerve conduit has following problems. Preparation and storage of the collagen is complicate since the collagen of the nerve conduit has to be extracted from animals. Producing large quantity of the collagen is not suitable and the cost of the nerve conduit is very expensive. These disadvantages limit utilizing the nerve conduit in clinics. In addition, the collagen-based nerve conduit has fast velocity of dissolution in the body; causes immune reaction and infectious disease; and has weak tensile strength in the body. For the synthetic polymer nerve conduit such as polylactic acid (PLA), polylactic acid-co-glycolic acid (PLGA), etc., which was proved its biocompatibility, although the nerve conduit has excellent structure safety and tensile strength, due to polymer tubular shape thereof, the body fluid can not be easily exchangeable and inflammation can be occurred because the biodegradation period is longer than 2 years and the breakdown products are acidic.
In this regard, the inventors of the present invention have researched a new kind of nerve conduit which can be used as a replacement of the existing synthetic polymers nerve conduit and found that silk nano-fiber nerve conduit has excellent biocompatibility, allows the body fluid to be exchanged in and out of the conduit through pores of the conduit, and has a proper elasticity, tensile strength and tear strength. Also, since the nerve conduit remarkably regenerates injured nerves to recover a motor skill and sensory function of the injured nerves, the inventors concluded that silk nano-fiber nerve conduit can be used to treat injured nerves as a replacement of the existing synthetic polymers nerve conduit and completed the present invention.